Import Illumos changes to address the following Illumos issues:
4976 zfs should only avoid writing to a failing non-redundant
top-level vdev
4978 ztest fails in get_metaslab_refcount()
4979 extend free space histogram to device and pool
4980 metaslabs should have a fragmentation metric
4981 remove fragmented ops vector from block allocator
4982 space_map object should proactively upgrade when feature
is enabled
4984 device selection should use fragmentation metric
MFC after: 2 weeks
0f4faf42cb